The present invention relates generally to liquid crystal displays (LCDs), and more particularly to a system, apparatus and method for directly driving LCD glass with a switched mode digital logic circuit.
Liquid crystal displays (LCDs) are commonly used in consumer electronics such as digital thermostats, alarm control panels, sprinkler system control panels, alarm clock radios, kitchen and laundry appliances, etc. LCDs act in effect as light valves, i.e., they allow transmission of light in one state, block the transmission of light in a second state, and some include several intermediate stages for partial transmission of light. The LCD comprises a thin layer of xe2x80x9cliquid crystal materialxe2x80x9d deposited between two plates of glass, and is often referred to as xe2x80x9cglass.xe2x80x9d Electrodes are attached to the inside (facing) sides of the plates of the glass. One electrode is referred to as common or backplane, and the other electrodes making up alpha-numeric and/or graphical images on the LCD are referred to as segments or pixels. Segments and pixels will be used herein interchangeably and will designate the LCD electrodes closest to the viewing surface of the LCD, e.g., between the backplane electrode(s) and the front of the LCD.
The LCD operates by applying a root mean square voltage (VRMS) between the backplane electrode and the pixel electrode. When a VRMS level of zero volts is applied to the LCD, the LCD is substantially transparent. To turn a LCD pixel xe2x80x9con,xe2x80x9d which makes the pixel turn dark or opaque, a VRMS level greater than the LCD threshold voltage is applied to the LCD. Different LCD material have different characteristics but all have in common a minimum RMS voltage that produces 90% contrast, Von, and a maximum RMS voltage that produces 10% contrast, Voff. Contrast is maximized when the LCD pixel is at its darkest or most opaque.
Many LCDs are multiplexed, that is, they have multiple common lines (also called backplanes) for a given set of segment connections. The timing pattern of sequentially selecting all of the backplanes is called a multiplex frame. Since the commons must multiplex or time-share their LCD segment data on the segment lines, the instantaneous voltage across these segments must be increased. Most LCD driver applications use charge-pump circuits to boost the voltage across the LCD pixels; this technology along with resistor ladders, allow LCD glass to be driven by multiple voltage sources.
LCD drivers have a high voltage level, Voh, and a low voltage level, Vol. When an LCD has just one backplane, the RMS voltage between the backplane and the segment(s) would be equal to Vohxe2x88x92Vol of the drivers. This is true because all segment(s) of the LCD glass would be constantly driven all the time. But if there are multiple backplanes, all segments cannot be driven concurrently. In order to adhere to the RMS Von spec of the LCD, when a given backplane of a segment(s) must be driven, then a greater voltage must be applied thereto. This is the reason why charge-pumps are traditionally used when driving LCD glass.
Notice that RMS voltages are specified on LCDs. This is an important requirement; LCD""s require zero DC offset. Even a small DC offset (usually greater than 50 mV) across any LCD material can damage that material. In order to keep the LCD glass xe2x80x98unpolarizedxe2x80x99, all of the asserted signals applied to an LCD must be reversed continually. The polarities between the backplane(s) and segments are typically changed after every multiplex frame. So, each positive frame is followed by a negative one, etc.
Another technique used by LCD designers is multiple voltage levels, also known as bias. These bias voltages allow the asserted segments in a multiplexed LCD to be driven while the deasserted ones remain at a voltage too low to affect them. A xc2xd bias drive would consist of two voltage levels above ground; or, Voh, and a mid-level voltage. A ⅓ bias drive would have a fourth voltage level (e.g., two voltages between Voh and Vol). And, a xc2xc bias drive would have three mid-voltage levels equally spaced between Voh and Vol. And so on for other bias ratios. Charge pumps are often also used to generate a greater supply voltage which, through the use of resistor ladder networks, create the desired middle voltages.
The asserted common line is at either Voh or Vol (depending if this is a positive or negative multiplex frame). The segment lines are brought to either Voh or Vol so as to produce the segment pattern desired. The non-asserted common lines must also have a certain voltage value for proper operation of the LCD. If the voltage driven on them was Voh or Vol, some other (e.g., non-selected) pixels would be affected since the segment lines are being driven. The unused commons cannot be left floating because a DC bias can result on the deasserted ones (e.g., one leg of these capacitors are tied to the pixel lines being driven and the other legs are tied to a floating common). The solution is to bring the deasserted commons to a mid-voltage. This voltage must be such that the voltage across a deasserted segment is LOWER than Voff and the voltage across an asserted segment is HIGHER than Von. When there are multiple backplanes present, sometimes it is easier to implement this with higher order bias ratios (⅓, xc2xc, ⅕, ⅙ or more).
More detailed descriptions of LCD operation and technologies are disclosed in Application notes AN563 and AN658 by Microchip Technologies Inc., 2355 West Chandler Blvd., Chandler, Ariz. 85224-6199. These application notes are incorporated herein by reference for all purposes.
Because of consumer product cost constraints, ease of manufacture, miniaturization, improved reliability, etc., it is desirable for a digital circuit (e.g., microcontroller, microprocessor, programmable logic array (PLA), application specific integrated circuit (ASIC) and the like) to directly drive LCD glass. An added benefit would be the ability to directly drive LCD glass having a plurality of backplanes and be able to also control contrast of the LCD without additional hardware components or manual adjustments.
What is needed is a system, method and apparatus for directly driving LCD glass with digital logic while retaining the capabilities of using multiplexed multiple backplanes with associated pixels and, in addition, being able to control LCD segment or pixel contrast.
The invention overcomes the above-identified problems as well as other shortcomings and deficiencies of existing technologies by providing hardware and software methods, and an apparatus for directly driving liquid crystal display (LCD) glass with a digital logic circuit (e.g., microcontroller, microprocessor, programmable logic array (PLA), application specific integrated circuit (ASIC) and the like). Software programs, firmware in EEPROM, mask ROM or a hardwired state machine, etc., may be used for control of the digital logic circuit. An exemplary software program for a microcontroller is attached hereto as xe2x80x9cAppendix Axe2x80x9d and is incorporated herein by reference for all purposes.
Advances in LCD technology allow the design engineer to specify lower voltage chemistries in their LCD displays and thus avoid using costly charge-pump circuits and power consumptive resistor ladders in their designs. This can be done via switch-mode techniques that need only a single supply voltage. The digital logic circuit has a plurality of digital outputs coupled to the backplane(s) and pixels of the LCD glass and functions as a xe2x80x9cswitched modexe2x80x9d LCD driver having the following features: 1) Substantially no DC bias of the LCD glass by continually reversing voltage polarity across the LCD material, 2) maintaining minimum refresh rate so as to avoid flicker, 3) the resultant RMS voltage across a deasserted pixel is less than Voff, and 4) the resultant RMS voltage across an asserted pixel is greater than Von.
Low power and voltage, e.g., 3.3 volts or lower, product applications using a microcontroller and an LCD will especially benefit from the present invention. For example, battery operated devices do not require power consuming charge pumps or resistor networks when using the embodiments of the present invention.
LCD contrast may be controlled by adjusting the time interval of the phases wherein all segments have no RMS voltage potential The longer the LCD segments remain at a zero potential, the lower the bias on all of the segments, hence contrast is lowered.
According to the present invention, different segments may have different contrast or shading. This is accomplished as follows, for a high contrast segment more phases are at Von for that segment. For a lower contrast segment, less phases are at Von.
The present invention is directed to an apparatus for driving a liquid crystal display (LCD), said apparatus comprises a liquid crystal display (LCD) having N backplanes and a plurality of segments; and a microcontroller having a plurality of digital outputs, wherein the N backplanes and the plurality of segments of the LCD are coupled directly to the plurality of digital outputs of the microcontroller. Wherein N is a positive integer number. The microcontroller and LCD may be adapted to be powered from a battery power supply. The LCD driving voltages are pulses from the microcontroller having voltage amplitudes substantially the same as a supply voltage of the microcontroller. The microcontroller drives the LCD according to an LCD driver program. The LCD driver program controls the microcontroller to produce a series of pulses from the plurality digital outputs coupled to the N backplanes and the plurality of segments of the LCD so as to control the LCD. The LCD driver program controls amplitude and duration of the series of pulses from the microcontroller so that there is a continually reversing voltage polarity across the LCD material. The LCD driver program controls amplitude and duration of the series of pulses from the microcontroller so that there is substantially no noticeable flicker of the LCD. The LCD driver program controls amplitudes and duration of the series of pulses from the microcontroller so that a resultant RMS voltage across an asserted one of the plurality of segments is greater than Von. The LCD driver program controls amplitude and duration of the series of pulses from the microcontroller so that a resultant RMS voltage across a deasserted one of the plurality of segments is less than Voff. The LCD driver program varies some of the duration""s of the series of pulses from the microcontroller so as to control the segment contrast of the LCD. The LCD driver program varies some of the amplitudes of the series of pulses from the microcontroller so as to control contrast between the plurality of segments of the LCD. A temperature sensor may be coupled to the microcontroller, wherein the temperature sensor supplies ambient temperature information to the microcontroller so that the LCD operating parameters may be adjusted for changes in the ambient temperature.
The present invention is also directed to a system using a microcontroller and a liquid crystal display (LCD), said system comprising: a liquid crystal display (LCD) having N backplanes and a plurality of segments; a microcontroller having a plurality of digital outputs, wherein the N backplanes and the plurality of segments of said LCD are coupled directly to some of the plurality of digital outputs of said microcontroller; and a control program for controlling said microcontroller, wherein said microcontroller performs a function and controls said LCD. The LCD may display parameters and information relating to the function.
The function performed by the microcontroller may include, but is not limited to, control of temperature (thermostat), humidity, sprinkler, alarm and security system, alarm clock, timer, clothes dryer, washing machine, toaster, microwave, oven, cooktop, clothes iron, water heater, tankless water heater, solar heating, swimming pool, jacuzzi, answering machine, pager, telephone, intercom, caller identification, electronic address book, treadmill, stationary bicycle, exercise machine, torque wrench, depth gauge, scale, speedometer, automobile tire condition status, anti-skid and anti-lock brakes, fuel gauge, engine monitoring, operation of luminaries (lights) in a building, power load management, video cassette player, DVD player, uninterruptable power supply (UPS), dictaphone, tape recorder, MP3 music player, video game toy, calculator, personal digital organizer, etc.
The present invention is further directed to a method of operation for driving a liquid crystal display (LCD) having a backplane and a plurality of segments, said method comprising the steps of:
applying a high level to a backplane, a low level to asserted ones of a plurality of segments, and the high level to deasserted ones of the plurality of segments for a first time period;
applying the high level to the backplane, the low level to the asserted ones of the plurality of segments, and the high level to the deasserted ones of the plurality of segments for the first time period;
applying the high level to the backplane, the low level to the asserted ones of the plurality of segments, and the low level to the deasserted ones of the plurality of segments for the first time period;
applying the low level to the backplane, the low level to the asserted ones of the plurality of segments, and the low level to the deasserted ones of the plurality of segments for a second time period;
applying the low level to the backplane, the high level to the asserted ones of the plurality of segments, and the low level to the deasserted ones of the plurality of segments for the first time period;
applying the low level to the backplane, the high level to the asserted ones of the plurality of segments, and the low level to the deasserted ones of the plurality of segments for the first time period;
applying the low level to the backplane, the high level to the asserted ones of the plurality of segments, and the high level to the deasserted ones of the plurality of segments for the first time period; and
applying the high level to the backplane, the high level to the asserted ones of the plurality of segments, and the high level to the deasserted ones of the plurality of segments for the second time period.
The present invention is also directed to a method of operation for driving a liquid crystal display (LCD) having N backplanes and a plurality of segments, said method comprising the steps of:
a) applying a high level to an asserted backplane i of N backplanes, the high level to deasserted backplanes of the N backplanes, the low level to asserted ones of a plurality of segments, and the high level to deasserted ones of the plurality of segments for a first time period;
b) applying the high level to the asserted backplane i of the N backplanes, the low level to the deasserted backplanes of the N backplanes, the low level to the asserted ones of the plurality of segments, and the high level to the deasserted ones of the plurality of segments for the first time period;
c) applying the high level to the asserted backplane i of the N backplanes, the low level to the deasserted backplanes of the N backplanes, the low level to the asserted ones of the plurality of segments, and the low level to the deasserted ones of the plurality of segments for the first time period;
d) applying the low level to the asserted backplane i of the N backplanes, the low level to the deasserted backplanes of the N backplanes, the low level to the asserted ones of the plurality of segments, and the low level to the deasserted ones of the plurality of segments for a second time period;
e) applying the low level to the asserted backplane i of the N backplanes, the low level to the deasserted backplanes of the N backplanes, the high level to the asserted ones of the plurality of segments, and the low level to the deasserted ones of the plurality of segments for the first time period;
f) applying the low level to the asserted backplane i of the N backplanes, the high level to the deasserted backplanes of the N backplanes, the high level to the asserted ones of the plurality of segments, and the low level to the deasserted ones of the plurality of segments for the first time period;
g) applying the low level to the asserted backplane i of the N backplanes, the high level to the deasserted backplanes of the N backplanes, the high level to the asserted ones of the plurality of segments, and the high level to the deasserted ones of the plurality of segments for the first time period;
h) applying the high level to the asserted backplane i of the N backplanes, the high level to the deasserted backplanes of the N backplanes, the high level to the asserted ones of the plurality of segments, and the high level to the deasserted ones of the plurality of LCD segments for the second time period; and
i) incrementing i by 1 then repeating steps a) through h) until i=N.
The present invention is also directed to a method of operation for driving a liquid crystal display (LCD) having N backplanes and a plurality of segments, said method comprising the steps of:
a) applying a low level to an asserted backplane i of N backplanes, the low level to deasserted backplanes of the N backplanes, the low level to asserted ones of a plurality of segments, and a high level to deasserted ones of the plurality of segments for a first time period;
b) applying the low level to the asserted backplane i of the N backplanes, the high level to the deasserted backplanes of the N backplanes, the low level to the asserted ones of the plurality of segments, and the high level to the deasserted ones of the plurality of segments for the first time period;
c) applying the low level to the asserted backplane i of the N backplanes, the high level to the deasserted backplanes of the N backplanes, the low level to the asserted ones of the plurality of segments, and the low level to the deasserted ones of the plurality of segments for the first time period;
d) applying the low level to the asserted backplane i of the N backplanes, the low level to the deasserted backplanes of the N backplanes, the low level to the asserted ones of the plurality of segments, and the low level to the deasserted ones of the plurality of segments for a second time period;
e) applying the high level to the asserted backplane i of the N backplanes, the high level to the deasserted backplanes of the N backplanes, the high level to the asserted ones of the plurality of segments, and the low level to the deasserted ones of the plurality of segments for the first time period;
f) applying the high level to the asserted backplane i of the N backplanes, the low level to the deasserted backplanes of the N backplanes, the high level to the asserted ones of the plurality of segments, and the low level to the deasserted ones of the plurality of segments for the first time period;
g) applying the high level to the asserted backplane i of the N backplanes, the low level to the deasserted backplanes of the N backplanes, the high level to the asserted ones of the plurality of segments, and the high level to the deasserted ones of the plurality of segments for the first time period;
h) applying the high level to the asserted backplane i of the N backplanes, the high level to the deasserted backplanes of the N backplanes, the high level to the asserted ones of the plurality of segments, and the high level to the deasserted ones of the plurality of LCD segments for the second time period; and
i) incrementing i by 1 then repeating steps a) through h) until i=N.
The present invention is also directed to a method of operation for driving a liquid crystal display (LCD) having N backplanes and a plurality of segments, said method comprising the steps of:
a) applying a high level to an asserted backplane i of N backplanes, the high level to deasserted backplanes of the N backplanes, a low level to asserted ones of a plurality of segments, and the high level to deasserted ones of the plurality of segments for a first time period;
b) applying the high level to the asserted backplane i of the N backplanes, the low level to the deasserted backplanes of the N backplanes, the low level to the asserted ones of the plurality of segments, and the high level to the deasserted ones of the plurality of segments for the first time period;
c) applying the low level to the asserted backplane i of the N backplanes, the low level to the deasserted backplanes of the N backplanes, the low level to the asserted ones of the plurality of segments, and the low level to the deasserted ones of the plurality of segments for a second time period;
d) applying the low level to the asserted backplane i of the N backplanes, the low level to the deasserted backplanes of the N backplanes, the high level to the asserted ones of the plurality of segments, and the low level to the deasserted ones of the plurality of segments for the first time period;
e) applying the low level to the asserted backplane i of the N backplanes, the high level to the deasserted backplanes of the N backplanes, the high level to the asserted ones of the plurality of segments, and the low level to the deasserted ones of the plurality of segments for the first time period;
f) applying the high level to the asserted backplane i of the N backplanes, the high level to the deasserted backplanes of the N backplanes, the high level to the asserted ones of the plurality of segments, and the high level to the deasserted ones of the plurality of segments for the second time period; and
g) incrementing i by 1 then repeating steps a) through f) until i=N.
The present invention is also directed to a method of operation for driving a liquid crystal display (LCD) having a backplane and a plurality of segments, said method comprising the steps of:
a) applying a low level to an asserted backplane i of N backplanes, a high level to deasserted backplanes of the N backplanes, the low level to asserted ones of a plurality of segments, and the high level to deasserted ones of the plurality of segments for a first time period;
b) applying the low level to the asserted backplane i of the N backplanes, the low level to the deasserted backplanes of the N backplanes, the low level to the asserted ones of the plurality of segments, and the high level to the deasserted ones of the plurality of segments for the first time period;
c) applying the low level to the asserted backplane i of the N backplanes, the low level to the deasserted backplanes of the N backplanes, the low level to the asserted ones of the plurality of segments, and the low level to the deasserted ones of the plurality of segments for a second time period;
d) applying the high level to the asserted backplane i of the N backplanes, the low level to the deasserted backplanes of the N backplanes, the high level to the asserted ones of the plurality of segments, and the low level to the deasserted ones of the plurality of segments for the first time period;
e) applying the high level to the asserted backplane i of the N backplanes, the high level to the deasserted backplanes of the N backplanes, the high level to the asserted ones of the plurality of segments, and the low level to the deasserted ones of the plurality of segments for the first time period;
f) applying the high level to the asserted backplane i of the N backplanes, the high level to the deasserted backplanes of the N backplanes, the high level to the asserted ones of the plurality of segments, and the high level to the deasserted ones of the plurality of segments for the second time period; and
g) incrementing i by 1 then repeating steps a) through f) until i=N.
A technical advantage of the present invention is low cost and reduced number of parts.
Another technical advantage is operation at low operating voltage levels.
Another technical advantage is no external or additional parts are required to directly drive the LCD.
Still another technical advantage is correction of the LCD bias voltages based on temperature.
A feature of the present invention is directly driving LCD glass without resistor networks or charge pumps.
Another feature is software control of contrast and brightness of LCD segments.
Another feature is operation at low voltage and/or system voltage.
Another feature is adjustment of LCD bias voltages based on temperature.
Features and advantages of the invention will be apparent from the following description of the embodiments, given for the purpose of disclosure and taken in conjunction with the accompanying drawings.